The fabrication of semiconductor devices is at present generally agreed to require silicon of at least the following purity:
TABLE I ______________________________________ Purity Requirements For Polysilicon.sup.a Electronically active Parts per billion impurity atomic ______________________________________ Group III elements Less than 0.3 (B, Al, etc.) Group V elements Less than 1.5 (P, As, etc.) Heavy metals Less than 0.1.sup.b Carbon Less than 300.sup.c Oxygen Less than 50.sup.c All others Less than 0.001.sup.b ______________________________________ .sup.a See Proceedings of 3rd International Symposium on Silicon Materials, Science & Technology, Vol. 772, p. 18, Electrochemical Society .sup.b Activation analysis. .sup.c Limits of detection (infrared).
This extremely high degree of purity in respect to silicon is referred to in the industry as "electronic grade" purity. Silicon solar cells can utilize a somewhat lower degree of purity than other semiconductor devices, so-called "solar grade" purity being roughly about 1/100 as pure as electronic grade silicon. That is, solar grade silicon can have roughly 100.times. the maximum impurity levels noted above for electronic grade silicon. Solar grade purity corresponds to a resistivity of about 1.0 ohm cm or better, whereas electronic grade roughly corresponds to a resistivity of about 100 ohm cm or better.
Silicon of these very high purity levels is currently required in very large quantities; the usage of electronic grade purity silicon alone is in excess of 2500 metric tons annually. The cost of purifying silicon to these extremely high purity levels, in the large quantities needed, has been a major factor--if not, indeed, a major bottleneck--in the industry.
Such exceedingly high levels of purity are required because of the disastrous electronic affects of even low levels of certain impurities. The most critical of these are electronically active impurities, specifically the Group III elements boron and aluminum and the Group V elements phosphorus and arsenic.
Originally elemental silicon was purified to the requisite purity by zone refining of previously formed impure elemental silicon. The elemental silicon used for purification by the zone refining process was generally produced by high temperature and/or hydrogen reduction of a decomposable silicon-containing compound such as SiCl.sub.4, SiHCl.sub.3, or SiH.sub.4. As an incident to the decomposition of the silicon-containing compound, impurities present in the compound codeposit with the silicon formed from the compound and thus appear as impurities in it. The impure silicon is then purified by the zone refining step. For a description of silicon zone refining purification techniques, see "Zone Melting," William G. Pfann, Wiley and Sons, 1958.
Zone refining can produce silicon of very high purity from impure silicon. However, zone refining is a slow and expensive technique, requiring very high power input to produce the moving molten zone. Furthermore, due to the unfavorable segregation coefficient between boron and silicon, boron impurities are not removed from the silicon by zone refining as effectively as other electronically active impurities.
The method which is presently used commercially to produce polycrystalline silicon is based on the purification of trichlorosilane (SiHCl.sub.3) by repetitive distillation, followed by decomposition of the purified compound on a hot silicon filament. Trichlorosilane has a boiling point of 33.degree. C., and is commonly purified by vacuum distillation. It is a corrosive compound, which further complicates its purification. The hydrogen reduction of trichlorosilane takes place at a relatively high temperature, which is a disadvantage because of the higher energy required to sustain the chemical reaction. Another disadvantage is that trichlorosilane contains only 20% silicon by weight, so that a relatively large weight of that compound must be processed to obtain a desired weight of silicon.
The primary objective of this invention has been to provide a new and unique process whereby elemental silicon of extremely high purity can be produced on an industrial scale, at a cost competitive with or below that of other available techniques, without need for zone refining or any other purification of the elemental product once formed.